The present invention relates generally to optical fibers and in particular to narrow linewidth high-power continuous wave or quasi-continuous wave fiber lasers and amplifiers.
Stimulated Brillouin scattering (SBS) is a limiting factor in the evolution of fiber lasers and amplifiers towards higher power and is also harmful to optical communications signals. Fundamentally, SBS is a nonlinear process that describes the scattering of laser light by a travelling hypersonic acoustic grating. As a by-product, Doppler-shifted scattered light, known as Stokes light, is generated. The interaction of the Stokes light with the laser light induces electrostriction in the medium and thus tends to re-enforce the acoustic wave. In an optical fiber, momentum conservation requires the Stokes light to propagate in the opposite direction as the laser light. It is well-known in the art that once a certain amount of optical power is coupled into or is generated in the fiber, significant backscattered Stokes light is produced causing the performance of the fiber to degrade. The SBS process is characterized by a gain spectrum that determines the SBS response of the medium to the pump frequency. Measurements in silica fibers have established a Brillouin shift of approximately 16 GHz and a linewidth of approximately 40 MHz at a wavelength of 1064 nm. The effective SBS gain can be diminished through the use of a broad linewidth seed laser. However, several important applications including coherent beam combination for directed energy purposes, harmonic generation, lidar, and gravitational wave detection require the use of high power narrow linewidth optical fiber amplifiers and lasers. Therefore, there is a need for techniques that mitigate the SBS process.
A commonly-used approximate formula to calculate the SBS threshold in optical fibers was proposed in 1972 by R. G. Smith in Applied Optics 11, pp. 2489-2494:Pth≈21 Aeff/gBLeff  (1)where Aeff is the optical effective area, Leff is the effective interaction length, and where gB is the peak value of the Brillouin gain. Therefore, The SBS threshold can be increased by decreasing the effective length of the fiber, increasing the effective area, or somehow manipulating the Brillouin gain in the fiber. The increase in the SBS threshold through the decrease of length is limited by the gain requirements in the fiber. Moreover, increased gain per unit length through the use of higher concentrations of rare earth elements is beset with problems associated with photodarkening and solubility limits. Much work has been done to increase the effective area of the fiber through the use of large mode area (LMA) fibers. While conventional LMA fiber designs have been successful in delivering single mode power outputs exceeding 100 watts, there is general agreement that new approaches are required for further enhancement is of the power.
A variety of experimental efforts have been attempted or proposed to reduce the SBS threshold through the manipulation of the SBS gain via the fiber doping profile (e.g. U.S. Pat. Nos. 7,130,514 and 7,167,621). In U.S. Pat. No. 5,851,259 by Clayton et al., the SBS threshold is reduced by introducing a modulation in the tension applied to the fiber during the draw process. This idea was expanded on in U.S. Pat. No. 6,542,683 by Evans et al. as a permanent, non-uniform stress is imparted to the fiber core through non-uniform thermal expansion and viscosity profiles. The latter inventor shows that a simple modulation of tension during the draw process leads to a marginal increase in the SBS threshold. The technique is limited by the fact that a change in the draw tension leads to a change in the fiber diameter. The latter inventor did not envision a fiber which could be manufactured with polarization maintaining properties.
In 2004, Wessels et al. reported in Optics Express 12, pp. 4443-4448 on a 72 m-long fiber amplifier pumped with two seed lasers. The two seed signals were separated by twice the SBS shift. The Stokes generated light from one laser signal coupled into the second laser light, allowing the first laser signal to grow to twice the power level of a single seed amplifier. One significant drawback of this technique is the requirement that the two seed signals have to be precisely tuned. Another significant drawback is that at such a small frequency separation, a parasitic process known as four-wave mixing (FWM) is prominent, leading to the generation of several frequency sidebands. This broadening of the optical power spectrum precludes the application of this method to fiber laser applications that require well-defined spectra such as coherent beam combining.
While some of the foregoing patents and applications may describe techniques that can lead to improvement in the power output of narrow linewidth amplifiers, each can have limitations. Accordingly, there remains a need in the art for new methods that address prior deficiencies.